[dev] fix #6081: how to debug the plugin host

Signed-off-by: Anton Kosyakov <anton.kosyakov@typefox.io>

.vscode/launch.json

@@ -58,8 +58,8 @@
         "--port=3000",
         "--no-cluster",
         "--app-project-path=${workspaceRoot}/examples/browser",
-        "--no-app-auto-install",
-        "--plugins=local-dir:plugins"
+        "--plugins=local-dir:plugins",
+        "--hosted-plugin-inspect=9339"
       ],
       "env": {
         "NODE_ENV": "development"
@@ -75,6 +75,21 @@
       "internalConsoleOptions": "openOnSessionStart",
       "outputCapture": "std"
     },
+    {
+      "type": "node",
+      "request": "attach",
+      "name": "Attach to Plugin Host",
+      "port": 9339,
+      "timeout": 60000,
+      "stopOnEntry": false,
+      "smartStep": true,
+      "sourceMaps": true,
+      "internalConsoleOptions": "openOnSessionStart",
+      "outFiles": [
+        "${workspaceRoot}/packages/plugin-ext/lib/**/*.js",
+        "${workspaceRoot}/plugins/**/*.js"
+      ]
+    },
     {
       "type": "node",
       "request": "launch",

doc/Developing.md

@@ -34,6 +34,7 @@ For Windows instructions [click here](#building-on-windows).
      - [Debug the Electron example's backend](#debug-the-electron-examples-backend)
      - [Debug the Electron example's frontend](#debug-the-electron-examples-frontend)
      - [Debug IPC servers](#debug-ipc-servers)
+     - [Debug the plugin host](#debug-the-plugin-host)
  - [**Profiling**](#profiling)
      - [Profile the frontend process](#profile-the-frontend-process)
      - [Profile the backend process](#profile-the-backend-process)
@@ -256,6 +257,15 @@ Let assume you have to work for instance in the `@theia/navigator` extension. Bu
 In order to look up `server-name` run the backend server with `--log-level=debug` flag to enable logging of IPC servers instantiation.
 You should be able to see message of `[${server-name}: ${server-PID}]: IPC started` format, like `[nsfw-watcher: 37557] IPC started`.
 
+### Debug the plugin host
+
+  - Pass `--hosted-plugin-inspect=9339` arg to the backend server from the command line.
+    - Instead you can run `Launch Backend` launch configuration which is already preconfigured.
+  - Open the debug view and run the `Attach to Plugin Host` launch configuration.
+    - It connects to the plugin host if at least one extension is detected, otherwise it timeouts after 60s.
+    - If you want to debug the activation then enable `stopOnEntry` flag.
+  - Open the browser page.
+
 ## Profiling
 
  - Use Chrome devtools to profile both the frontend and backend (Node.js).